The present invention relates to an amplification optical fiber for use in optical communications and an optical fiber amplifier, which are suited to the case of reducing differential modal gain.
In optical communications, signals are superposed on a light beam propagated through an optical fiber for communications. In order to increase information volumes transmitted through one core in such optical communications, few-mode communications are known, in which information is superposed on a light beam in an LP01 mode (a fundamental mode) of a signal light beam and information is also superposed on a light beam in a mode higher than the LP01 mode for information communications. Therefore, in the case of amplifying a signal light beam propagated through an optical fiber using an optical fiber amplifier, it is necessary to amplify light beams in individual modes propagated through the optical fiber.
Non-Patent Literature 1 describes an amplification optical fiber like this. In the amplification optical fiber described in Non-Patent Literature 1, the refractive index of its core is a step type, i.e. the refractive index in the inside of the core is constant in the radial direction. The core is doped with erbium. The concentration of erbium is constant in the inside of the core. However, in the case of amplifying light beams using this amplification optical fiber, the difference between the gain of the LP01 mode light beam and the gain of the LP11 mode light beam, i.e. differential modal gain (DMG), is sometimes increased.
Therefore, Non-Patent Literature 2 describes an amplification optical fiber aimed for reducing such differential modal gain. The amplification optical fiber described in Non-Patent Literature 2 has the refractive index profile of its core similar to that of the amplification optical fiber described in Non-Patent Literature 1. However, the center region including the center axis of the core is not doped with erbium, and the outer circumferential region surrounding the center region is doped with erbium. The power of the LP01 mode light beam is made equal to the power of the LP11 mode light, beam propagated through the region to which erbium is doped, and thus the gain of the LP01 mode light beam is matched with the gain of the LP11 mode light beam.
[Non-Patent Literature 1] Y. Yung et al., “First demonstration of multimode amplifier for spatial division multiplexed transmission systems,” Proc. ECOC '11, Th.13.K4 (2011)
[Non-Patent Literature 2] G. Le Cocq et al., “Modeling and characterization of a few-mode EDFA. supporting four mode groups for mode division multiplexing,” Opt. Express 20, 27051-27061 (2012)